A stud bump structure and method for manufacturing the same are provided. The stud bump structure includes a substrate, and a first silver alloy stud bump disposed on the substrate, wherein the first silver alloy stud bump has a weight percentage ratio of ag:Au:Pd=60-99.98:0.01-30:0.01-10.
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1. A stud bump structure, comprising
a first substrate; and
a first silver alloy stud bump disposed on the first substrate and assembled to a second substrate, wherein the first silver alloy stud bump is composed of ag, Au, and Pd and has a weight percentage ratio of ag:Au:Pd=89.99-97.99:0.01:2-10,
wherein the first substrate comprises a wafer and the second substrate comprises a wafer or a printed circuit board.
2. The stud bump structure as claimed in
3. The stud bump structure as claimed in
4. The stud bump structure as claimed in
5. The stud bump structure as claimed in
6. The stud bump structure as claimed in
7. A method for manufacturing a stud bump structure as claimed in
providing a silver alloy wire;
melting an end of the silver alloy wire to form a first free air ball;
bonding the first free air ball onto a first substrate to form a first ball bond;
cutting off the silver alloy wire, such that the first ball bond is left on the first substrate to form a first silver alloy stud bump, wherein the first silver alloy stud bump is composed of ag, Au, and Pd and has a weight percentage ratio of ag:Au:Pd=89.99-97.99:0.01:2-10; and
assembling the first silver alloy stud bump to a second substrate,
wherein the first substrate comprises a wafer and the second substrate comprises a wafer or a printed circuit board.
8. The method for manufacturing a stud bump structure as claimed in
9. The method for manufacturing a stud bump structure as claimed in
10. The method for manufacturing a stud bump structure as claimed in
11. The method for manufacturing a stud bump structure as claimed in
melting an end of the silver alloy wire again to form a second free air ball;
bonding the second free air ball onto the first silver alloy stud bump to form a second ball bond; and
cutting off the silver alloy wire, such that the second ball bond is left on the first silver alloy stud bump to form a second silver alloy stud bump.
12. The method for manufacturing a stud bump structure as claimed in
13. The method for manufacturing a stud bump structure as claimed in
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This Application claims priority of Taiwan Patent Application No. 101141253, filed on Nov. 7, 2012, the entirety of which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to a stud bump structure and a method for manufacturing the same, and in particular, relates to a silver alloy stud bump structure and a method for manufacturing the same.
2. Description of the Related Art
Interconnection methods for integrated circuit chips include wire bonding, tape automated bonding, flip chip bonding, or the like. In a tape automated bonding device or a flip chip bonding devices, a conductive stud bump is used as a connection point between a chip and a substrate. These techniques are all high density packaging techniques.
Advantages of flip chip bonding may include having great amounts of connection points, small distances between each connection point, small package areas, good performance at a high frequency, high reliability, and good resistance to electromagnetic interference. Therefore, flip chip bonding has been commonly used in packaging processes for an electric device such as an integrated circuit (IC) or a light emitting diode (LED).
Manufacturing and packaging a bump play an important role in a flip chip bonding process. Most of the flip chip bumps are made of solder alloys, such as Sn-37Pb, Sn-9Zn, Sn-0.7Cu, Sn-3.5Ag, Sn-51In, Sn-58Bi, Sn-3-Ag-0.5Cu, Sn-9Zn-3Bi, or the like.
Methods for manufacturing a solder bump may include electroplating and stencil printing. However, a solder bump manufactured by electroplating is usually harmful for the environment and its specific alloy composition is usually difficult to control. In addition, it is also difficult to find an appropriate plating solution and plating process to form a Pb-free solder bump therein. For example, if a bump is formed of an alloy such as Sn-3.5Ag, Sn-0.7Cu, or Ag-0.5Cu, the composition of the alloy is usually hard to control. If a bump is formed of an alloy such as Sn-51In, Sn-58Bi, or Sn-9Zn—Bi, it is usually very difficult to find an appropriate plating solution.
Therefore, nowadays, stencil printing of a solder paste has become an essential method in a flip chip packaging process. A key material for a flip chip solder paste is tin powders. Generally, a particle size of a tin powder in a surface mount technology (SMT) is between about 30 μm and 50 μm, wherein the tin powder of this size is easier to be manufactured. However, since a size of a flip chip bump is usually smaller than 120 μm, the size of the tin powder is required to be smaller than 10 μm and the tin powder with this small size is very difficult to be manufactured. In addition, when the size of the flip chip bump is decreased to be smaller than 100 μm, or even about 50 μm, each bump may only contain a few tin powders even if the size of the tin powder is smaller than 10 μm. Therefore, the difficulty of coplanarity tends to occur after a reflow process. Other problems of manufacturing a flip chip bump by solder paste include holes being formed by flux after a reflow process and manufacturing failures of the stencil printing may increase when a distance between each connection point is less than 100 μm.
In electronic packaging industry, electroplating gold and copper bumps are also used for flip chip assembly. However, the electroplating process for gold and copper bumps has also concern of environmental pollution. In addition, the intermetallic compounds grow rapidly at the interface between gold bump and aluminum pad, which are often accompanied with the occurrence of Kirkendall voids. Both the effects of intermetallic compounds and Kirkendall voids can cause the embrittlement and electrical resistance increase of connection point. In contrast, the intermetallic compounds formed at the copper bump/aluminum pad interface are very thin, leading to an insufficient bonding. An alternative bumping method, using gold or copper wires to produce gold or copper stud bumps, has similar disadvantages of excessive intermetallics growth for gold stud bump and insufficient intermetallics formation for copper stud bump. The rigidity of copper stud bump can additionally cause the cracking of under-pad chip.
Tape automated bonding (TAB) is another technique used in a high density packaging process. Advantages of the TAB package may include having a great amount of connection points, high strength of connection points, and good electrical and thermal conductivity. In addition, the tape automated bonding may be performed automatically and can be tested during packing process. Furthermore, the thickness of a resulting package may be minimized. In a TAB process, gold and copper bumps are conventionally employed, which are also manufactured by electroplating or stud bumping method as in a flip chip process. However, either electroplating gold bump or gold stud bump is expensive due to the material cost and the problems of excessive intermetallic compounds and Kirkendall voids similarly occur in a TAB process as in a flip chip assembly. On the other hand, either electroplating copper bump or copper stud bump in TAB package has the disadvantages of insufficient intermetallics growth and severe oxidation. The failure of under-pad chip cracking due to the rigidity of copper can also be caused during the copper stud bumping process.
An embodiment of the disclosure provides a stud bump structure, including: a substrate; and a first silver alloy stud bump disposed on the substrate, wherein the first silver alloy stud bump has a weight percentage ratio of Ag:Au:Pd=60-99.98:0.01-30:0.01-10.
Another embodiment of the disclosure provides a method for manufacturing a stud bump structure, including: providing a silver alloy wire; melting an end of the silver alloy wire to form a first free air ball (FAB); bonding the first free air ball onto a substrate to form a first ball bond; and cutting off the silver alloy wire, such that the first ball bond is left on the substrate to form a first silver alloy stud bump, wherein the first silver alloy stud bump has a weight percentage ratio of Ag:Au:Pd=60-99.98:0.01-30:0.01-10.
A detailed description is given in the following embodiments with reference to the accompanying drawings.
The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
Moreover, the formation of a first feature over and on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact.
Referring to
Referring to
Referring to
According to one embodiment, the first silver alloy stud bump 208 has a weight percentage ratio of Ag:Au:Pd=60-99.98:0.01-30:0.01-10, wherein silver is the main element in the first silver alloy stud bump 208 (for example, containing more than 50 wt % of silver). According to the experiments, it was discovered that if a stud bump is formed of pure silver (Ag), the silver stud bump will be too soft and easily oxidize. However, if an appropriate amount of gold (Au) is added into the stud bump, the bonding strength of the stud bump and the oxidation resistance will be improved. In addition, if an appropriate amount of palladium (Pd) is added into the stud bump, it can not only improve the strength and oxidation resistance of the stud bump, but also suppress excessive formation of intermetallic compounds and electromigration of silver. On the other hand, if too much gold is added into the stud bump, the interface between stud bump and aluminum pad may contain too thick intermetallic compounds, resulting in embrittlement of the connection point and an increase of costs. In addition, when too much palladium is added into the stud bump, the stud bump may become too rigid and too brittle, limiting its applications.
It is noted that although the stud bump structure described above comprises specific compositions of silver, gold, and palladium, the scope of the disclosure is not to be limiting. According to some embodiments, the stud bump structure may also comprise other metal elements, non-metal elements, or some contaminants. However, these additional elements should be used depending on various applications, and the performance of the stud bump structure should remain the same.
Compared to stud bumps formed of other metals (such as a gold stud bump or a copper stud bump), the silver alloy stud bump according to various embodiments can be more widely applied to various applications and have a better stability. For example, if gold (Au) stud bump is used, a great amount of gold in the gold stud bump may be dissolved into melting solder to form an intermetallic compound, AuSn4, when the stud bumping chip is assembled to a substrate by soldering process. Furthermore, when the device is operated or when the reliability test is performed, a great amount of brittle intermetallic compounds, AuxAly, and Kirkendall voids may be formed at the interface between gold stud bump and aluminum pad, resulting in a breakage of the connection points, an increase of electrical resistively, and a decrease of the reliability of the electronic products. Therefore, generally, the gold stud bump is assembled by a polymer conductive adhesive. However, the electrical and thermal conductivities of polymer conductive adhesive is much worse than metallic solder bonding.
On the contrary, if a copper stud bump is used, there may be insufficient intermetallic compounds formed at the interface between copper stud bump and aluminum pad, and therefore it will be difficult to know whether or not it is connected successfully. In addition, resulting products tend to fail when a metal residue test is required. Furthermore, copper is prone to oxidize and corrode, and the reliability of resulting electric devices is usually poor. Moreover, since rigidity of copper is high, the under-pad chip may be broken during the copper stud bumping process. Therefore, it is difficult to use a copper stud bump in the process described above.
On the other hand, the silver alloy stud bump according to various embodiments is softer, and therefore, a chip may not be damaged when bonding the free air ball thereon. There may not be so much brittle intermetallic compounds as the gold stud bump formed during the bonding process. In contrast to the copper stud bump, there may still be enough amount of the intermetallic compounds formed, leading to a satisfactory bonding. In addition, the resulting products can pass the metal residue test. According to various embodiments, the silver alloy stud bump may be bonded by soldering, conductive adhesive, or hot pressing, but other bonding methods may also be used depending on applications.
In addition, the silver alloy stud bump may be formed on a chip or may be directly formed on a wafer to perform a wafer level package. An advantage of forming the silver alloy stud bumps on a wafer directly includes the processing cost being reduced by forming a great amount of the silver alloy stud bumps before cutting the wafer. In a wafer level packaging process, more than ten thousands of stud bumps may be formed on a single wafer before dicing. In other words, the wafer is continuously heated during the hot pressing process to form the stud bumps. Therefore, when the last stud bump is finally formed on the wafer, the first stud bump formed at the beginning of the process may be heated with the wafer for a long time. As a result, the early bonded gold stud bumps have a concern that a great amount of intermetallic compounds may grow after the finish of wafer level packaging process. In this case, the gold stud bump formed at the initial stage of the process may be damaged due to the embrittlement of the connection point and the bonding strength (ie. adhesive force) of the connection points being weakened. Therefore, the gold stud bump has encountered severe problems for the application in wafer level packaging. In contrast, copper stud bumps may result in insufficient intermetallic compounds at the bump/pad interface and are not suitable for a wafer level package, either. In addition, copper is so rigid that the wafer may be damaged during the bonding process. More severely, copper tends to be oxidized at high temperatures, especially for those stud bumps formed at the beginning of the process and have been heated with the wafer for a long time during the wafer level packaging.
On the other hand, for the silver alloy stud bumps, the intermetallic compounds may be grown slowly to an appropriate thickness and the connection can be confirmed by the appearance of the intermetallic compound. In addition, when the silver alloy stud bumps are formed directly on a wafer during wafer level packaging, even if the silver alloy stud bumps formed at the initial stage of the process have been heated with the wafer for a long time, there will not be a great amount of intermetallic compounds formed resulting from heating. Therefore, the bonding strength of the silver alloy stud bumps can be maintained. It is obvious that the silver alloy stud bump in this invention is superior to the gold stud bump and copper stud bump for the application in wafer level package.
Referring to
The stud bump stacking structure 212 formed by the first silver alloy stud bump 208 and the second silver alloy stud bump 210 has a height greater than an original one, and therefore the electric devices assembles by the stud bump stacking structure may have a higher tolerance for deformation, leading to better reliability. In addition, since the first silver alloy stud bump 208 and the second silver alloy stud bump 210 may be formed by the same silver alloy wire 200, the structure may be formed in a single manufacturing machine. Therefore, the process may be simplified and the cost may be decreased.
According to one embodiment shown in
According to one embodiment shown in
According to one embodiment shown in
According to one embodiment shown in
According to various embodiments described above, a novel silver alloy stud bump and a method for manufacturing the same are provided. The silver alloy stud bump may form a sufficient amount of intermetallic compounds with an aluminum pad, such that a satisfactory bonding interface may be resulted in. On the other hand, since the intermetallic compound of the silver alloy stud bump grows very slowly, the interface of the connection points will not become brittle, and the reliability of the silver alloy stud bump may be improved. In addition, compared to solder bumps, the silver alloy stud bump is better for the environment (resulting in less pollution) and the composition of the silver alloy may be controlled more precisely. Moreover, the difficulty of coplanarity may not occur. Further, the silver alloy stud bump may be used in a wafer level packaging process.
One end of a pure gold wire (diameter: 25.4 μm) was melted by electric flame off (EFO) to form a free air ball due to surface tension. The free air ball was bonded to an aluminum pad on a silicon chip to form a ball bond by ultrasonic hot pressing. Then, the gold wire was cut off to form a gold stud bump. The manufacturing details are shown in Table 1. The size of the resulting gold stud bumps are shown in Table 2. The adhesion force of the resulting gold stud bumps are shown in Table 3.
One end of a pure copper wire (diameter: 25.4 μm) was melted by electric flame off (EFO) to form a free air ball due to surface tension. The free air ball was bonded to an aluminum pad on a silicon chip to form a ball bond by ultrasonic hot pressing under forming gas (95% N2+5% H2). Then, the copper wire was cut off to form a copper stud bump. The manufacturing details are shown in Table 1. The size of the resulting copper stud bumps are shown in Table 2. The adhesion force of the resulting copper stud bumps are shown in Table 3.
One end of a silver alloy wire (89 wt % Ag-8 wt % Au-3 wt % Pd; diameter: 25.4 μm) was melted by electric flame off (EFO) to form a free air ball due to surface tension. The free air ball was bonded to an aluminum pad on a silicon chip to form a ball bond by ultrasonic hot pressing. Then, the silver alloy wire was cut off to form a silver alloy stud bump. The manufacturing details are shown in Table 1. The size of the resulting silver alloy stud bumps are shown in Table 2. The adhesion force of the resulting silver alloy stud bumps are shown in Table 3. Referring to Table 2, the size of the silver alloy stud bump was smaller than the size of the gold stud bump and slightly larger than the size of copper stud bump although the wires used for forming the stud bumps had the same diameter. The smaller size of the silver alloy stud bump had an advantage as it may be used in a high density package with a small connection distance. A ball shear test was performed to the resulting silver alloy stud bump in Example 1 and the gold stud bump in Comparative Example 1 by a bond tester DAGE4000, and the result showed that the adhesion force of the silver alloy stud bump was 21% and 28% higher than the adhesion force of the gold stud bump and copper stud bump, respectively.
TABLE 1
The manufacturing details
Silver alloy stud
Gold stud
Copper stud
bump
bump
bump
Power
230
(dac)
230
(dac)
260
(dac)
Time
10
(ms)
13.5
(ms)
13.5
(ms)
Adhesion force
24
(gf)
24
(gf)
30
(gf)
Adhesion time
12
(ms)
12
(ms)
15
(ms)
Current
46
(mA)
40
(mA)
50
(mA)
Discharge time
0.46
(ms)
0.46
(ms)
0.46
(ms)
TABLE 2
The size of resulting alloy stud bumps
Silver alloy stud
Gold stud
Copper stud
bump
bump
bump
Diam-
Thick-
Diam-
Thick-
Diam-
Thick-
eter
ness
eter
ness
eter
ness
Sample number
30
30
30
30
30
30
Maximum
52.5 μm
17.2 μm
55.4 μm
17.6 μm
52.3 μm
19.5 μm
Minimum
46.6 μm
15.4 μm
47.0 μm
15.8 μm
45.8 μm
15.2 μm
Average
49.3 μm
16.0 μm
53.4 μm
16.2 μm
48.8 μm
15.4 μm
TABLE 3
The adhesion force of resulting stud bumps
Silver alloy stud
Gold stud
Copper stud
bump
bump
bump
Sample number
30
30
30
Maximum
32.22 g
26.35 g
28.3 g
Minimum
27.12 g
21.08 g
20.1 g
Average
29.42 g
24.32 g
23.0 g
The silver alloy stud bump in Example 1, gold stud bump in Comparative Example 1, and copper stud bump in Comparative Example 2 manufactured on silicon chips were respectively bonded to a chip and then underfilled and flip-chip assembled to a bismaleimide triazine resin (BT resin) substrate of a ball grid array package (BGA) by soldering process (solder composition: 96.5 wt % Pb-3 wt % Ag-0.5 wt % Cu). Then, the free air ball was disposed on the back side of the BT substrate to finish the assembly of a flip chip/BGA high density packaging product. After that, a temperature cycling test (TCT), a pressure cooking test (PCT), a high temperature storage (HTS) test were performed to each of the resulting stud bump packaging products to test their reliability.
According to the experiments, the intermetallic compound formed between the interface between the gold stud bump and the aluminum pad on the chip had a thickness of 2.1 μm. The intermetallic compound was so thick that the connection points of the product became brittle and broken, resulting in a failure of the product. On the other hand, the intermetallic compound formed between the interface of the copper stud bump and the aluminum pad had a thickness of 0.2 μm. The amount of the intermetallic compound was too little to know whether the connection was successful or not. However, the intermetallic compound formed at the interface between the silver alloy stud bump and the aluminum pad had a thickness of 0.6 μm. The intermetallic compound was thick enough to know whether the connection was completed, but it was not too thick to make it break easily.
In addition, 500 times of the temperature cycling test were performed to the gold stud bump in Comparative Example 1, and the thickness of the intermetallic compound of the gold stud bump increased to 3.8 μm. 3000 times of the temperature cycling test were performed to the copper stud bump in Comparative Example 2, and the thickness of the intermetallic compound of the copper stud bump was only 0.3 μm. 3000 times of the temperature cycling test were performed to the silver alloy stud bump in Example 1, and the thickness of the intermetallic compound of the silver alloy stud bump increased to 1.0 μm.
Furthermore, after 168 hours of the pressure cooking test, the thickness of the intermetallic compound of the gold stud bump in Comparative Example 1 increased to 3.1 μm, the thickness of the intermetallic compound of the copper stud bump in Comparative Example 2 was only 0.4 μm, and the thickness of the intermetallic compound of the silver alloy stud bump in Example 1 increased to 0.9 μm.
Furthermore, after 500 hours of the high temperature storage test, the thickness of the intermetallic compound of the gold stud bump in Comparative Example 1 increased to 4.3 μm, such that the aluminum pad on the chip was almost consumed completely, the thickness of the intermetallic compound of the copper stud bump in Comparative Example 2 was only 0.7 μm, and the thickness of the intermetallic compound of the silver alloy stud bump in Example 1 increased to 1.8 μm.
Accordingly, the intermetallic compound of the gold stud bump in Comparative Example 1 grew so fast that the connection points became brittle. On the contrary, there was not enough of an intermetallic compound grown to know whether the connection was completed or not for the copper stud bump in Comparative Example 2. However, the intermetallic compound of the silver alloy stud bump in Example 1 had the thickness between the thickness of the intermetallic compound of the gold stud bump and the copper stud bump. In other words, the interface of the silver alloy stud bump did not become brittle and the connection was completed. Therefore, the silver alloy stud bump in Example 1 had a better reliability than the stud bumps in Comparative Examples 1 and 2.
12,000 of the silver alloy stud bumps in Example 1, gold stud bumps in Comparative Example 1, and copper stud bumps in Comparative Example 2 were formed on a silicon wafer (6 inches), respectively, to test for a wafer level package.
After all the stud bumps were formed on the wafer, the thickness of the intermetallic compound formed thereon was analyzed. According to the experiments, the intermetallic compound formed in the first ten gold stud bumps (ie. the first stud bump to the tenth stud bump formed at the beginning of the process) had an average thickness of 3.2 μm, the intermetallic compound formed in the first ten copper stud bumps had an average thickness of 0.4 μm, and the intermetallic compound formed in the first ten silver alloy stud bumps had an average thickness of 0.8 μm. The gold stud bumps formed at the beginning of the process became brittle and broke after all the stud bumps were formed. The copper stud bumps were poorly connected and were seriously oxidized, and the chip almost broke. However, the silver alloy stud bump did not have the problems described above.
Furthermore, the yields of the gold stud bumps in Comparative Example 1 and the silver alloy stud bump in Example 1 were almost 100%. However, the yield of the copper stud bumps in Comparative Example 2 was about 64%.
In addition, an average adhesion force of the first ten gold stud bumps (ie. the first ten stud bumps formed at the initial stage of the process) was 17% lower than an average adhesion force of the last ten gold stud bumps. An average adhesion force of the first ten copper stud bumps was 37% lower than an average adhesion force of the last ten copper stud bumps. An average adhesion force of the first ten silver stud bumps was almost the same with an average adhesion force of the last ten silver alloy stud bumps.
The silver alloy stud bump in Example 1, the gold stud bump in Comparative Example 1, and copper stud bumps in Comparative Example 2 were formed on an aluminum pad on an integral circuit chip respectively. A hot pressing head was used to bond the stud bump and a copper electrode on a polyimide (PI) substrate. Then, a ball shear test was performed to the silver alloy stud bump and the gold stud bump on the chip by a bond tester DAGE4000. The results are shown in Table 4. As shown in Table 4, the adhesion force of the silver alloy stud bump was 17% and 18% higher than the adhesion force of the gold stud bump and copper stud bump, respectively.
TABLE 4
Results of the shear test performed to the silver
alloy stud bump and the gold stud bump
Silver alloy stud
Au stud
Copper stud
bump
bump
bump
Sample number
30
30
30
Maximum value
63.2 g
54.6 g
58.3 g
Minimum value
58.9 g
50.4 g
49.7 g
Average value
61.2 g
52.5 g
51.9 g
While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
Lee, Jun-Der, Tsai, Hsing-Hua, Chuang, Tung-Han
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